Redox amplification processes have been described, for example in British Specification No. 1,268,126 and U.S. Pat. Nos. 3,748,138, 3,822,129 and 4,097,278. In one aspect of such processes, color materials are developed to produce a silver image (which may contain only small amounts of silver) and are then treated with a redox amplifying solution to form an amplified dye image.
The developer solution contains a color developing agent and the amplifier contains an oxidizing agent that will oxidize the color developing agent in the presence of the silver image that acts as a catalyst.
Oxidized color developer reacts with a color coupler to form the image dye. The amount of dye formed depends on the time of treatment or the availability of color coupler and is less dependent on the amount of silver in the image as is the case in conventional color development processes.
Examples of suitable oxidizing agents include peroxy compounds including hydrogen peroxide and compounds that provide hydrogen peroxide, e.g., addition compounds of hydrogen peroxide.
Published PCT Application WO 92/07299 describes an example of such a process in which the photographic material is processed in separate color developer and amplification baths. The color developer solutions described therein have a pH of 10.3 and contain potassium carbonate as alkaline agent. On standing such a color developer darkens indicating that it is not very stable.
U.S. Pat. No. 4,084,969 describes a process in which the redox oxidant is a Co(III) complex. In Examples 1-3 the photographic material is processed in separate color developer and amplification baths, the color developer solutions having a pH of 10.0-10.1 and contain potassium carbonate as alkaline agent. In Example 4 the photographic element was exposed to X-rays using a fluorescent screen and the color developer solution is said to have a pH of 11.5. The alkaline agent was again potassium carbonate. No improvement in performance or stability is reported.
There is a constant desire to improve redox amplification processes that use a peroxide amplifier to provide better sensitometric results and more stable processing solutions.